1. Field of Invention
The invention relates to a method for carrying out endothermic and exothermic chemical reactions.
2. Description
A great many chemical reactions are characterized by a positive heat effect (exothermic reaction) or a negative heat effect (endothermic reaction). To enable chemical reactions to proceed in the desired manner, an efficient supply or removal of the reaction heat is indispensable. In some exothermic reactions, the thermodynamic equilibrium shifts in an undesired direction if the temperature rises. Examples are the synthesis of ammonia and methanol, the oxidation of sulfur dioxide to sulfur trioxide in the production of sulfuric acid, the reaction of sulfur dioxide with hydrogen sulfide in the Claus process, the selective oxidation of H2S to elementary sulfur and the reaction of carbon monoxide with hydrogen to methane. Since in the course of these reactions thermal energy is released, the temperature of the reaction mixture will rise and the thermodynamic equilibrium will shift in an unfavorable direction, unless the reaction heat released is removed fast and efficiently from the reactor.
In endothermic reactions too, a shift of the thermodynamic equilibrium in an undesired direction can occur, now by the consumption of thermal energy. Examples are methane-steam reforming and the dehydrogenation of ethylbenzene to styrene. A problem may also arise in that as a result of the consumption of energy by the reaction, the temperature of the reaction mixture decreases unduly, so that the desired reaction no longer proceeds.
Not only can a temperature change cause a shift of the thermodynamic equilibrium in an unfavorable direction, it can also adversely affect the selectivity of catalytic reactions.
Examples of reactions where the temperature affects the selectivity are the production of ethylene oxide from ethylene (the undesired reaction is the formation of water and carbon dioxide), the selective oxidation of hydrogen sulfide to elementary sulfur (the undesired reaction is the formation of SO2) and the Fischer Tropsch synthesis. In all cases, a temperature rise occurs as a result of the release of the reaction heat. If this temperature rise is not prevented through a rapid removal of the reaction heat, the selectivity decreases greatly.
In most conventional catalytic reactors, use is made of a fixed bed of catalyst particles. In such a catalyst bed, porous bodies of catalyst particles have been poured or piled.
In order to avoid an undesirably high pressure drop across such a catalyst bed, it is preferred to use bodies or particles of dimensions of at least 0.3 mm. These minimum dimensions of the catalyst bodies are necessary to keep the pressure drop that occurs upon the passage of a stream of reactants through the catalyst bed, within technically acceptable limits. While the dimensions are limited at the lower end of the range by the permissible pressure drop, the necessary activity of the catalyst imposes an upper limit on the dimensions of the catalytically active particles. The high activity required for a number of types of technical catalysts can mostly be achieved only with a surface of the active phase of 25 to 500 m2 per ml catalyst volume. Surfaces of such an order of magnitude are possible only with very small particles, for instance with particles of 0.05 xcexcm. Since particles with such dimensions no longer allow a liquid or gas mixture to flow through them, the primary, extremely small particles have to be formed into high-porous bodies with dimensions of at least about 0.3 mm, which can possess a large catalytic surface. An important task in the production of technical catalysts is to combine the required high porosity with a sufficiently high mechanical strength. The catalyst bodies cannot be allowed to disintegrate upon filling of the reactor and upon exposure to sudden temperature differences (thermal shock).
Under the conditions of the thermal pretreatment and/or catalytic reaction to be carried out, nearly all catalytically active materials soon sinter to form large conglomerates with a negligibly small active surface. Therefore, the active component (finely divided) is generally applied to a so-called support. This support exhibits the necessary thermal stability and hardly sinters, if at all, at high temperatures. Often used as supports are silicon dioxide, aluminum oxide or activated carbon.
As appears from the above examples, there is a very great need for a fast supply or removal of thermal energy in catalytic reactors, but the total heat transfer coefficient is mostly very low in a fixed catalyst bed. According to the present state of the art, it is virtually impossible to supply thermal energy to or remove it from a fixed catalyst bed in an efficient manner. This is indeed evident from the manner in which chemical reactions are carried out in fixed catalyst beds.
It is possible that of an exothermic reaction only the thermodynamic equilibrium shifts in adverse direction upon a temperature rise, without the selectivity decreasing unallowably. In that case, the reaction in a fixed catalyst bed can be made to proceed adiabatically. After passage through the reactor, the stream of reactants is cooled off in a separate heat exchanger. Because the conversion of the reactants is now thermodynamically limited by the temperature rise in the reactor, the unconverted reactants have to be reacted again upon cooling. The reaction product can be separated and the reactants can be recycled through the fixed catalyst bed. This occurs, for instance, in the ammonia and methanol synthesis. If the reaction product cannot be easily separated, downstream of the heat exchanger a second fixed bed reactor with a heat exchanger must be linked up. This is for instance the case in the oxidation of sulfur dioxide to sulfur trioxide. Sometimes, to prevent emission of harmful compounds, even a third reactor with heat exchanger is necessary. If connecting a number of reactors and heat exchangers in series is not properly possible and the separation of the reaction product is not either, the reaction product is sometimes recirculated through the catalyst bed. Per passage through the reactor, so little of the reactants is added to the circulating reaction product that it is converted completely. Because the rise of the temperature must then be properly controlled, per passage through the reactor only very little can be converted. In cases where the reaction must be carried out at a greatly increased pressure, the problems with the supply or removal of the reaction heat are extra large.
In the ammonia synthesis and the methanol synthesis, a catalyst bed is employed in which reactants are injected at different points at a relatively low temperature.
Such an implementation of the method, whereby gas streams must be passed through high pressure reactors in a complicated manner, obviously also requires high investments.
In a number of technically important cases, it is desired in catalytic reactions to work with a high to very high spatial throughput rate, with a great pressure drop across the reactor being considered a less serious drawback. In the conventional fixed bed reactors, a high pressure drop with the corresponding high spatial throughput rate is not properly possible. If the pressure at the reactor inlet is increased, the catalyst may be blown (gaseous reactants) or washed (liquid reactants) out of the reactor. It is also possible that at a particular critical value of the pressure at the reactor inlet xe2x80x9cchannelingxe2x80x9d occurs. In that case, the catalyst particles in a particular part of the reactor are going to move. In that case, the reactants are found to flow virtually exclusively through the part of the catalyst bed that is in motion.
With the fixed bed reactors current at present, the catalyst bed clogs up. Therefore the reactor must be regularly opened and the cumulated layer of dust removed. It would be favorable if a pulse of gas of high pressure could be sent through the reactor in a direction opposite to that of the stream of reactants. This pressure pulse would blow the dust off the catalyst bed; thus, clogging could be prevented without opening the reactor, which is technically very attractive. With the fixed bed catalysts according to the present state of the art, however, this is not possible; along with the dust, the catalyst bodies would be blown out of the catalyst bed.
It will be clear that a number of disadvantages can be associated with the use of fixed bed reactors. In general, it requires costly facilities, while recirculation and separation of reaction products present in low concentration require a great deal of energy. For that reason, in a number of cases a whirling bed is employed. In a whirling bed the transport of thermal energy is much easier, while the problems with pressure drop and clogging do not occur. In a whirling bed, however, the catalyst to be used must meet very high standards regarding mechanical strength and wear resistance, which is not at all possible with every catalyst. Finally, the catalyst consumption in a whirling bed is relatively high due to the unavoidable wear. Accordingly, in many cases it will not be possible to use a whirling bed.
There are cases where it is not possible to work either with a whirling bed or with an adiabatic reactor. This applies in particular to highly endothermic reactions and reactions where the selectivity decreases unallowably upon increase of the temperature. Examples are methane-steam reforming and the selective oxidation of ethylene to ethylene oxide. In a selective oxidation of ethylene, a very large heat exchanging surface is employed by utilizing a reactor with no less than 20,000 long tubes. In methane-steam reforming it is attempted to optimize the heat supply and to limit the pressure drop by adjusting the dimensions and the shape of the catalyst bodies. In this last reaction too, a large number of costly tubes have to be used in the reactor.
It has also been proposed to apply the catalyst exclusively to the wall of the reactor. An example of such a system is described in the abstract of JP-A 6/111838. According to this publication, a reform catalyst has been provided in grooves of a plate, while in grooves of a second plate a combustion catalyst has been provided. These plates have been arranged against each other, so that through the heat generated with the combustion the reforming can take place.
Also in carrying out the Fischer Tropsch reaction, in which from a mixture of hydrogen and carbon monoxide higher hydrocarbons are produced, a system has been employed, in which a catalyst is provided on the wall of the reactor. This catalyst provided on the wall ensures a good heat transfer from the catalyst to the outside of the reactor. For providing the catalyst on the wall, inter alia the following method has been proposed. The catalyst is applied as a Raney metal, an alloy of the active metal and aluminum. After being applied, the catalyst is activated by dissolving the aluminum with lye. The greater part of the reactor volume is empty, as a result of which the contact between the reactants and the catalytically active surface is slight and the conversion per passage through the reactor is greatly limited. The reactants must therefore be frequently recirculated through the reactor.
In a number of technically important cases, the pressure drop upon passage of the reactants through the catalyst bed must remain very low. This applies, for instance, to reactors in which flue gas of large plants is to be purified, as with the catalytic removal of nitrogen oxides from flue gas. Because a flue gas stream is generally very large, a substantial pressure drop requires a very great deal of mechanical energy. The same applies to the purification of exhaust gases of automobiles. In this case too, a high pressure drop is unallowable.
Currently, the use of catalysts provided on a honeycomb is one of the few possibilities of achieving an acceptable pressure drop without unallowably reducing the contact with the catalyst. To that end, often ceramic honeycombs (honeycombs, monoliths) are used, in which the catalytically active material has been provided.
A variant of the method in which the catalyst is provided exclusively on the wall, is the use of monoliths made up of thin metal sheets. Such a reactor is manufactured, for instance, by rolling up a combination of corrugated and flat thin metal sheets and subsequently welding them together. It is also possible to stack the flat sheets in a manner leading to a system with a large number of channels. On the wall of the thus-obtained channels the catalyst is then provided.
As has been noted, the thermal conduction in a fixed catalyst bed is poor. This has been ascribed to the low thermal conductivity of the high-porous supports on which the catalytically active material has been provided. Therefore Kovalanko, O. N. et al., Chemical Abstracts 97 (18) 151409u have proposed to improve the thermal conduction by increasing the conductivity of the catalyst bodies. They did this by using porous metal bodies as catalyst support. Now, it has already been described by Satterfield that the thermal conductivity of a pile of porous bodies is determined not so much by the conductivity of the material of the bodies, as by the contacts between the bodies among themselves (C. N. Satterfield, xe2x80x9cMass Transfer in Heterogeneous Catalysisxe2x80x9d, MIT Press, Cambridge, Mass., USA (1969), page 173). The inventors"" own measurements have shown that the thermal conductivity of catalyst bodies indeed does not greatly affect the heat transport in a catalyst bed.
In WO-A 86/02016 a reactor is described, comprising a reaction bed provided with a catalyst, which bed consists of sintered metal particles which are in good heat conducting communication with the reactor wall, which wall is externally provided with sintered metal particles for removing reaction heat. Further, on the outside of the reactor a phase transition occurs. Such a reactor system is found to be able to realize a large heat dissipation, but has the disadvantage that a good setting and/or control of the reaction is not possible, or very difficult. This is evident inter alia from the example in which the catalytic combustion of a combustible gas with a heat of combustion of 35.530 kJ/m3 is described. This would have to occur at a temperature of 350xc2x0 C. However, by the cooling of the reactor with evaporating water (steam production) at 110xc2x0 C., the entire reactor is cooled to 110xc2x0 C., so that the reaction will not occur.
In U.S. Pat. No. 4,101,287 a combined heat exchanger reactor is described, consisting of a monolith, through a part of the channels of which flow the reactants and through a part of which flows the cooling agent. Here the same disadvantage as in the system of WO-A 8602016 presents itself.
In EP-A 416710 a method is described, based on the use of a catalytic reactor in which the reactor bed consists of elementary particles of metal sintered to each other and to one side of the reactor wall, while no sintered metal particles are present on the other side of the reactor wall. When in such a reactor the diameter of the reactor bed is chosen in relation to the heat effects, which vary from one reaction to another, but are known and, depending on the reaction conditions, can be calculated, reactions of the type referred to can be carried out optimally.
In carrying out chemical reactions, especially if they are reactions which are carried out on a large scale, if a strong heat effect is involved, or if high pressures are required, problems accruing from the heat economy of the reaction are encountered. It appears that in a number of cases it is not easy to efficiently supply the necessary heat or remove the heat produced. For instance in steam-reforming natural gas or other hydrocarbons, the necessary amount of heat is so large that complex systems with burners and heating tubes are needed to supply the necessary heat. This kind of problems also occurs with other reactions with great thermal effects, such as the production of ammonia, the preparation of ethylene oxide, the selective oxidation of H2S, and the like. Even in reactors according to the above-mentioned EP-A 416710 or U.S. Pat. No. 4,101,287, it is found this can give rise to problems.
Presently, mixtures of hydrogen and carbon monoxide (synthesis gas) are produced through reaction of methane with steam, the so-called methane-steam reforming process. If only hydrogen is desired, the carbon monoxide is allowed to react with steam to form carbon dioxide and hydrogen. The carbon dioxide formed is removed through dissolution under pressure in aqueous solutions or regenerable solid sorbents.
For this process, it is possible to use, besides methane, other gaseous hydrocarbons or naphtha or other hydrocarbons that can be readily brought into the gas phase.
To enable the highly endothermic reaction between methane and steam to proceed, the necessary reaction heat must be supplied to the reaction mixture at a high temperature, for instance 850xc2x0 C. (allothermic process). In general, the necessary heat is generated outside the reaction mixture by combustion of, for instance, methane. In order to transfer the thus generated thermal energy to the reaction mixture, a partition with a sufficiently high thermal conductivity must be used.
Through radiation the reaction heat generated in the combustion reaction is transferred to the reaction mixture. The reaction mixture is passed through tubes of a high-grade alloy in which a suitable catalyst has been provided. The tubes are exposed to the radiation of the burners.
To prevent oxidation of the tubes at the necessary high temperatures, costly (nickel-containing) alloys must be used for the reactor wall. To save energy, the methane-steam reforming process is often carried out at elevated pressure, for instance at 30 bar, which imposes even more stringent requirements of oxidation resistance.
For the steam-reforming mentioned, it has previously been proposed to carry out a partial oxidation of the hydrocarbon prior to the reaction. The heat thereby produced is then stored in the gas and carried to the steam reforming. Such a method, however, cannot be employed in all cases.
The object of the invention is to provide a suitable method for carrying out chemical reactions and in particular chemical reactions with a great thermal effect, whereby in a simple manner the transport of the necessary or redundant heat is provided for.
The invention is based on the surprising insight that it is possible to optimally adjust the heat conduct of combined endothermic and exothermic reactions to each other if use is made of a specific reactor system in which at least two reactor beds based on porous structures are in heat exchanging relation with each other.
The invention accordingly relates to a method for carrying out two chemical reactions in a reactor system comprising at least two mutually separate reactor beds, of which the surfaces exposed to the reactants are catalytically active for the chemical reactions concerned, and at least one partition, wherein
at least one first reactor bed is present, which is bounded by at least one partition, which bed is based on a continuous porous structure and which bed is fixedly connected to said partition,
at least one second bed is present, which is based on a continuous porous structure, and which bed is fixedly connected to said partition, and
said second bed, with respect to the first bed, is disposed on the other side of said partition, so that a heat exchanging contact between said beds is present and the reaction heat of a first chemical reaction which is carried out in said first reactor bed is supplied or absorbed by carrying out a second chemical reaction in said second bed.
Over the above-described known systems, the invention has the advantage that during the process a greater stability is obtained, inter alia because of the self-regulating character of the system.